The present invention relates in general to fusion bonding or thermo bonding of two or more workpieces and, more particularly, to methods and apparatus characterized by their ability to automatically form, on a continuous, reproducible, high speed, production-line basis, fusion bonds devoid of structural, electrical and/or cosmetic defects between two or more workpieces by a technique hereinafter referred to as "positive displacement bonding." In its principal aspects, the invention is concerned with improved methods and apparatus for automatically moving a heated electrode through the portions of the workpiece(s) to be bonded so as to uniformly heat and melt those portions of the workpiece(s) to be bonded while, at the same time, displacing the molten material into a storage area or reservoir surrounding the heated electrode where such material is maintained in its uniformly heated molten state while further movement of the electrode into the workpiece(s) creates a cavity therein and, thereafter, retracting the heated electrode so as to permit the molten material to return to the cavity formed by the electrode in the workpiece(s) where such molten material is allowed to cool and solidify, thus forming a flawless bond between the workpieces.
In recent years, there has been an ever increasing trend toward, and demand for, automation and mechanization in virtually all branches of industry. In many industries, typically including, but not limited to, the battery making industry, it is often necessary to bond two or more workpieces together to form a unitary assembly wherein the bond is characterized by its structural stength and/or, in some instances, by excellent characteristics of electrical conductivity. Various methods have been devised for bonding such workpieces together including, merely by way of example, welding, thermo-compression bonding, ultrasonic bonding, percussion welding, etc.
The particular technique selected has heretofore depended upon many variable parameters, including: (1) the sizes and/or shapes of the workpieces; and (2) various characteristics of the particular materials to be bonded which may vary widely in such areas as electrical and/or thermal conductivity characteristics, melting points, etc. Moreover, the particular technique employed has often been dictated by physical limitations in access to the region where the bonds are to be effected. Merely by way of example, in the battery industry it is often necessary to bond two or more pieces of lead together at various points, in some cases internally and in others externally of a given battery cell. Lead, of course, is characterized by having a relatively low melting point on the order of only 630.degree.F., as contrasted with, for example, steel which has a melting point on the order of 3,000.degree.F. Moreover, where the lead workpieces comprise battery straps, plates, terminal posts and/or intercell connectors, such as commonly employed in industrial motive-power batteries, automotive batteries, and the like, it is often difficult to gain access to the parts to be bonded. Even where access can be obtained, one is normally limited in the amount of heat that can be applied and in the types of reducing agents that can be utilized by virtue of other components present in the area of the bond to be effected such, for example, as the battery casing or cell jar which is commonly made of rubber, the electrolytic acids present in or to be added to the battery cells, the pasted positive and/or negative plates, the separators which are commonly made of microporous rubber, etc.
Many efforts have been made to devise improved bonding techniques which can be universally applied for the purpose of bonding two or more workpieces together irrespective of the wide range of variable parameters mentioned above. Moreover, consistent with the demands of industry today, numerous efforts have been made to devise bonding techniques which are capable of automation so as to enable automatic bonding of multiple workpieces as an integrated part of mass-production line and/or assembly line techniques. Typical of the aforementioned approaches are those described in U.S. Pat. Nos. 3,591,755, 3,608,809, and 3,706,126 of Robert Holbrook Cushman, assigned to the Western Electric Company, and relating to mechanical-thermal-pulse continuous fusion bonding processes and apparatus which are based, at least in part, upon a combination of applied and controlled pressure and temperature to effect a desired bond.
However, despite all such prior efforts which have met with varying degrees of success, certain industries have continued to employ the more tedious, time-consuming, manual bonding techniques which have been known and utilized for many years. Typical of these is the industrial motive-power battery industry where lead-to-lead bonds are still almost universally made by hand-torching or hand-burning techniques employing oxyacetylene torches and/or carbon burning tools. These techniques require highly skilled artisans who are capable of forming satisfactory bonds only after considerable training and, even then, a relatively high percentage of the bonds formed are not capable of meeting the rigorous quality control standards set by the battery industry. Typical of the types of difficulties encountered even by such skilled artisans are: (1) non-uniform heating of the interface between the parts to be bonded resulting in no bonding at all at some locations, and/or "burn-out" of connectors and/or other parts because of "over burning," thereby destroying the connector or other parts; (2) actual damage to and/or destruction of the rubber casing or battery cover due to inadvertent direct application of the flame or carbon tip thereto; (3) lack of control over, and resultant non-uniformity of the depth of, bond penetration into the parts to be bonded, thereby resulting in bonds which are unsatisfactory from either or both of structural and/or electrical conductivity characteristics; and (4) substantially complete melting of one of the two or more parts to be bonded accompanied by failure to melt the surface of a second of the pieces to be bonded, thereby resulting in a "cold-knit" between the properly and improperly melted pieces.
As a direct result of the inability of certain industries--for example, the battery industry--to utilize the aforementioned known automatic and semi-automatic bonding systems, and the continued industry-wide reliance on hand-torching and/or hand-burning techniques, numerous disadvantages have continued to plague such industries. More specifically: (1) various industries, at great expense to themselves, have had to continue to attempt to train personnel in the difficult, time-consuming hand-torching or hand-burning techniques; (2) as a result of the relatively high heat generated by such techniques, the use of low melting point, economical, lightweight plastic battery casings has been precluded; (3) the percentage of batteries and/or battery cells rejected because of unsatisfactory bonds has remained high; and (4) the number of batteries which have passed rigorous quality control tests and/or procedures while having latent defects in the bonds has been unacceptably high, resulting in customer dissatisfaction because of the presence of "leakers," particularly in the battery post/intercell connector, as well as an extremely objectionable phenomenon known in the art as "electro-capillary action " wherein battery electrolyte is actually pumped out of the battery cell through minute passages passing through the positive battery post/connector interface where the bond is defective, thereby not only weakening the cell affected and decreasing its life and usefulness, but often creating a direct short which drains the battery and which often causes corrosion and irreparable damage to other equipment in the immediate area.